Many liquid media possess a certain amount of elasticity and tensile strength, and are classified as viscoelastic materials. Some liquid media are characterized by pronounced elasticity and high tensile strength, and exhibit unique characteristics during laminar flow, i.e., superelastic liquids.
One characteristic of superelastic liquids is a tendancy to recoil when deforming forces acting on them are removed, and the potential energy stored in the liquids as a result of deforming forces is converted into kinetic energy. Another unique characteristic of superelastic liquid media is the ability to build up resistance to deformation as shear stresses increase and to flow readily at shear stresses about certain values.
It has been found that these and other properties qualify such liquid media as particularly useful for controlling lost circulation in oil field drilling operations and in related applications.
The superelastic liquids which have the greatest utility are those prepared by crosslinking water-soluble polymers in aqueous solution. It has been found that polysaccharidic hydrocolloids can be crosslinked in aqueous solution to produce viscous liquid media which are highly elastic and have considerable tensile strength.
U.S. Pat. No. 2,968,581 describes a method of crosslinking polymeric polyhydroxy materials which involves reacting the said materials in the presence of water with a preformed polymeric acetal.
U.S. Pat. Nos. 3,214,235 describes the production of crosslinked derivatives of polyhydroxy compounds by reaction with a diester of propiolic acid.
U.S. Pat. Nos. 3,215,634 describes the preparation of superelastic liquids which involves reacting an aqueous solution of a polysaccharide with a crosslinking agent which yields borate ions in aqueous solution. An acyclic C.sub.2 -C.sub.5 polyhydric alcohol is added in a concentration sufficient to react with excess borate ions and stabilize the crosslinked polysaccharide.
U.S. Pat. No. 3,697,498 describes a novel crosslinking agent for use with aqueous solutions of polysaccharides to form enhanced shear rate thinning viscoelastic solutions, wherein the said crosslinking agent consists essentially of a chelating composition formed from an organic polyelectrolyte ligand component, and a compound of a metal of the first series of the transition elements of the Periodic Table.
U.S. Pat. No. 3,836,465 describes a viscoelastic hydraulic fluid composition comprising an aqueous medium containing a mixture of polyalkylene oxide and lignosulfonate salt.
U.S. Pat. No. 4,183,765 describes a method of increasing the viscosity of a hydroxyalkyl cellulose solution by treatment with benzoquinone under controlled pH conditions.
The various non-Newtonian viscoelastic solutions of the type reviewed above usually are characterized by one or more disadvantages which limit their applicability in important commercial operations, e.g., for use as hydraulic fluid vehicles in well-drilling and in secondary oil and gas recovery from subterranean formations.
One disadvantage is that some high viscosity solutions are not sufficiently thixotropic in relation to low and high shear rates.
Another disadvantage is that with some methods of thickening aqueous solutions there is not sufficient control over the attainment and maintenance of the desired viscosity range and properties.
Another disadvantage of some viscoelastic solutions is an intolerance to the presence of inorganic electrolytes.
A further disadvantage of some viscoelastic solutions is a sensitivity to temperatures above about 60.degree. C., and a consequential degradation of rheological properties.
Accordingly, it is an object of this invention to provide a process for improving the viscoelastic properties of an aqueous medium.
It is another object of this invention to provide a novel thickening agent adapted to impart thixotropic properties to an aqueous medium.
It is another object of this invention to provide a thixotropic aqueous solution which exhibits improved heat stability and tolerance to the presence of inorganic electrolytes.
It is a further object of this invention to provide a process for derivatizing polygalactomannan gum contained in leguminosae endosperm splits.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.